design details for a butter knife warmer [Archive]

Ivan Seeking

Sep26-04, 01:38 AM

You need to read this.

http://www.sci-journal.org/index.php?template_type=report&id=26&htm=reports/vol3no1/v3n1k44.html&link=reports/home.php&c_check=1

and this
http://www.the12volt.com/ohm/ohmslaw.asp

By controlling the watts [power produced] you will control the heat produced. More watts means more heat. You don't want to mess with AC power [110 volts] especially without help from an adult who is knowledgeable about electricity.

I was thinking that you may not even need the nichrome wire. Maybe you could just use a resistor. Why don't you try this as a place to start. Get a 10 ohm, 1/4 watt resistor. You can get these at Radio Shack. Use one D cell to power the resistor. This will overload the battery a bit so don't run it very long; say no longer than one, five, ten, then thirty seconds at a time at first. Check the battery frequently and make sure that it doesn't get hot. You will probably see the battery voltage drop as a result of the overload. The resistor will get hot and should produce about 0.2 watts of power. See if this will do the job. Then you can think about your next step.

You should work with an adult who is knowledgeable about electricity. For one, the battery can overheat and even explode. Always wear safety goggles.

EDIT: Does anyone know the internal resistance [ESR] of a typical D cell?

Shay10825

Sep29-04, 10:42 AM

Would I have the end of the resistor just touching the battery and the other end not touching anything?

The D cell has an internal resistance of about 3 Ω

Cliff_J

Sep29-04, 01:34 PM

Shay the resistor is the heating element. It needs to have current passing through it to heat up. So the resistor is on the butter knife and then wires connect each end of it to the battery.

To find the power, consult ohm's law. Use algerbra to solve the following equasions:

Voltage = Current * Resistance
Power = Voltage * Current

Then you could weigh the butter knife and find the grams of steel in it. Now what is the heat capacity of the steel in the butterknife? How about the butter that will coat the blade (could probably use water to get close) as its cutting? That should be in your report as to how you determined the resistance and power level.

Where did you get the ESR figure from for a D cell? Sounds wayyyyyyy too high as I found 3 miliohms in a quick google search.
Cliff

Shay10825

Sep29-04, 01:54 PM

Since I'm using a D cell battery how would I connect the wires to the battery? For example if the D cell battery was in the battery compartment of a flash light the ends of the battery would be touching the metal spring and the metal plate. Would I connect the resistor wires to the spring and the plate?

Would it work with a D cell battery? Is there any way I could use a AA battery?

The resistor will be in the middle of two blades put together to make one blade. Will the resistor have to touch anything while in the blade? Since both ends of the resistor have to touch the battery the resistor will touch the battery, go up the blade, turn around, and touch the other end of the battery?

I found the ESR from a google search too. Mine could be wrong.

Cliff_J

Sep29-04, 02:36 PM

Battery sizes roughly give a clue of the power available.

You'll notice an AAA, AA, C, D battery are all 1.5V so voltage doesn't change. Instead its the current they can deliver that changes (determined by the ESR) and how long they can deliver that current (determined by the capacity).

Look at this chart:
http://www.techlib.com/reference/batteries.html

According to this if you use a D cell and used 12 amps (12,000 miliamps) it should last 1 hour, or if you used 6 amps it should last 2 hours. Mathimatically that's correct but remember our ESR from above? It will drastically reduce that and the more current you pull the less accurate the formula. A lot of batteries are rated for capacity at 20 hours so capacity at 1 hour its probably overrated by 5-10x. So with 12A from a D cell I'd guess maybe 6-12 mins before its dead and I might be optimistic.

Ok, back up from the construction for a second. Answer these questions:
How much heat power do you need?
How much voltage do you have with 1 cell? How about 2 cells?
How much resistance do you need to flow the correct amount of current from that voltage?
How long will each battery last flowing that current?

Yes, you will need the resistor to touch the blade. You'll likely want to cover the resistor leads with heatshrink or electrical tape. Then maybe some heatsink compound to assist in the heat transfer from resistor to blade.

Oh, and make sure the resistor has a rating higher than the number of watts you plan to use.

Cliff

Ivan Seeking

Sep29-04, 05:21 PM

Thanks for jumping in Cliff. I am fighting time right now...darned work! I have PF to attend to. :biggrin:

Oh, and make sure the resistor has a rating higher than the number of watts you plan to use.

But not so much higher that the resistor never gets hot [hence the 0.2 watt target]. Also, the knife will heat sink the resisitor so I was thinking that with good thermal coupling he may be able to overdrive things a bit. I was thinking that staying just below the rated power is a good place to start in order to insure a high max temperature.

Note: I just wanted to be sure that we weren't close to the ESR with a 10 ohm resistor. I thought it was more like 3 ohms for a D cell...hmmm...learn something new every day. Now, wait, that can't be right. At 1.5 volts and an ESR of milliohms, the total current available is way too high.

Shay10825

Sep30-04, 11:44 AM

Ok, back up from the construction for a second. Answer these questions:
How much heat power do you need?
How much voltage do you have with 1 cell? How about 2 cells?
How much resistance do you need to flow the correct amount of current from that voltage?
How long will each battery last flowing that current?

Cliff

Is this correct?:
~~~~~~~

Since the specific heat of silver, cal/g * degrees C, is .056 you change that into watts which is .234 W. So is .234 W the amount of heat power I need?
~~~~~~~

With one cell I have 1.5 volts, and with two clees I have 3 volts.
~~~~~~~

P=VC
.234= 1.5C
C=.156

V=CR
1.5=.156R
R=9.615

For 1.5 V I need 9.615 ohms of resistance?
~~~~~~~

P=VC
.234=3C
C=.078

V=CR
3=.078R
R=38.462

For 3 V I need 38.462 ohms of resistance?
~~~~~~~

With 1.5 V the battery will last 77 hours?
With 3 V the battery will last 154 hours?
~~~~~~~
Is any of this correct?

Cliff_J

Sep30-04, 04:15 PM

Good, you looked up specific heat. That was the point of the question, you can describe that different materials heat differently. You'll likely want to use steel instead of silver. I did a quick search and this site came up, pretty handy comparison:
http://www.ex.ac.uk/trol/dictunit/notes5.htm

Note that the level for water is much higher. You need to factor in how long you can wait and how much heat will be transmitted to the air from the knife to be exact, unnecessary in all likelyhood. But if instead you could heat up the butter knife first and then cut the butter you can take advantage of the thermal mass of the butter knife. Lets assume .2W is a nice starting point, its a safe level to begin experimenting.

Your math looks fine for all the rest. (As a side note, on the internet most people will use the standard letters E=voltage and I=current and R=resistance. Its a long story why....)

You can find resistors of sizes close to those values, like the 10 ohm mentioned by Ivan above or a 39 ohm resistor if you choose to use two cells. As you can also tell now that you've done the math (and should have a spiffy report with) you can see a 1/4 watt resistor will work fine.

As another side note, you can see that for the same power level a doubling of voltage means 4x the resistance is need to stay at the same power level. Or if you double the voltage and keep the resistance the same, you have 4x the power.

-----------------------------------------

Ivan - 3 ohms still sounds too high, even at .2A the drop is .6V? Sure .003 sounds way too low (how good is a unknown source on the internet) but maybe somewhere in the middle like 1 ohm? Maybe find time to crack open a flashlight this weekend and test a couple batteries. :smile:

Cliff

Ivan Seeking

Sep30-04, 05:16 PM

[COLOR=Blue]

Is this correct?:
~~~~~~~

Since the specific heat of silver, cal/g * degrees C, is .056 you change that into watts which is .234 W. So is .234 W the amount of heat power I need?

A couple of more details here. The specific heat is in calories per gram per degree centigrade. So, after converting calories to joules [not watts], you need 0.234 joules, per gram of silver, per degree C of temperature change for the silver mass.

Now, power - watts - is the number of joules of energy per second. The 0.234 watts calculated would raise one gram of silver, one degree C per second. Two grams of silver would heat at a rate of 0.5 degrees C per second, and so on. Also, as Cliff pointed out, this does not account for the heat lost.

Ideally we would want to include the mass and specific heat of the butter as well. This, plus the heat lost to the atmosphere would give us the total heat load, but you can probably ignore this for now. It should be mentioned in your report though.

Ivan Seeking

Sep30-04, 05:27 PM

If we see you selling this on QVC, Cliff and I will be waiting for our royalty checks. :biggrin:

Oh yes, there is another issue of how fast the resistor tranfers heat to the metal. As mentioned earlier, use heat sink paste [available at Radio Shack], but also make sure that the resistor [not the leads] is in direct contact with the metal blade. This will be a little tedious to set up properly but it is very important.

You may also find that the metal blade is able to dissipate the heat [hence power] faster than you can supply it. If this happens you will need to produce more power.

BoulderHead

Sep30-04, 05:55 PM

I get around this 'hard butter' problem by avoiding refrigeration.

Good luck

Ivan Seeking

Sep30-04, 06:05 PM

I get around this 'hard butter' problem by avoiding refrigeration.

Good luck

Actually, if you use real butter you can get minor food poisoning this way. Most such cases are misinterpreted as minor influenza or just a bad stomach.

BoulderHead

Sep30-04, 06:42 PM

I hadn't known that, but generally use the imitation stuff, if any at all.

megashawn

Sep30-04, 07:13 PM

Ivan Seeking

Sep30-04, 07:57 PM

I hadn't known that, but generally use the imitation stuff, if any at all.

I hate to guess what I've spent on Kaopectate due to my long, room temp butter habbit. It took me years to make the connection. I only realized the potential connection when I saw a woman from the National Centers for Disease Control who stated that something like 80% of all minor, alleged cases of influenza are really minor food poisoning.

Shay10825

Oct1-04, 02:47 PM

If I use two D cell batteries an tthe 39 ohm resistor will the blade heat up faster?

The 10 ohm .25 watt resistor is short in length. How would I have both ends of it touching the battery if it is soshort. Would I connect some kind of wire t theend of the resistor to make it longer?

Ivan Seeking

Oct1-04, 06:15 PM

If I use two D cell batteries an tthe 39 ohm resistor will the blade heat up faster?

No. In this case the power is about the same. Just try multiplying the voltage and current for each. In order to increase the power you would need less resistance or more voltage. Note that you can also get 1/2 watt resistors and higher.

The 10 ohm .25 watt resistor is short in length. How would I have both ends of it touching the battery if it is soshort. Would I connect some kind of wire t theend of the resistor to make it longer?

Yes. Make sure that you don't cause a short circuit. You might want to consider using an old flashlight for a battery holder.

Ivan Seeking

Oct1-04, 07:25 PM

hey, instead of making a heated knife, why not make a knife heater?

You know, some lil thing just barely wide enough to slide a standard butter knife in it. Then, use some heating elements to warm the knife. This way, you can warm any knife, instead of one special knife.

You may not have seen the original thread in GD where some similar suggestions were made. Shay10825 is hooked on a heated blade for now.

Gokul43201

Oct1-04, 10:23 PM

A couple of more details here. The specific heat is in calories per gram per degree centigrade. So, after converting calories to joules [not watts], you need 0.234 joules, per gram of silver, per degree C of temperature change for the silver mass.

Now, power - watts - is the number of joules of energy per second. The 0.234 watts calculated would raise one gram of silver, one degree C per second. Two grams of silver would heat at a rate of 0.5 degrees C per second, and so on. Also, as Cliff pointed out, this does not account for the heat lost.

Ideally we would want to include the mass and specific heat of the butter as well. This, plus the heat lost to the atmosphere would give us the total heat load, but you can probably ignore this for now. It should be mentioned in your report though.

Hmm...completely missed this thread, or I would have jumped in earlier.

Okay I don't want to be a spoiler, but here's a problem I raised in the original thread that I'll rephrase in the current context.

Say you have 0.23 W of power (1.5V battery with 10 ohm resistor).

For stainless, C ~ 0.5 J/K-gm.
For a blade that is pretty small (2"long, 1/2" wide and 1mm thick), the mass is about 4 gms (density ~ 8 gm/cc).
Assuming we need no more than a 10K rise in temperature, the required heat is H ~ 4*0.5*10 ~ 20 J

With losses (and keeping in mind that stainless is a pretty lousy conductor ~ 16 W/K-m) it looks like you'll need over 3 minutes to get the knife hot enough.

Is that okay ?

Shay10825

Oct1-04, 11:23 PM

The calculation I made earlier was for Sterling Silver. The Silver was what had the .23 W not stainless steal. I don't think I am going to be able to use silver so would it still take 3 min if I used Stainless Steal? Would I still be able to use the .25 W 10 ohm resistor for the steal or would I have to get a different resistor for it to heat up faster?

Ivan Seeking

Oct1-04, 11:32 PM

Okay I don't want to be a spoiler, but here's a problem I raised in the original thread that I'll rephrase in the current context.

My degree is in physics. That's an engineering problem. :biggrin:

I thought 1/4 watt was a nice place to start - that we can "see where to go from there".

Ivan Seeking

Oct1-04, 11:35 PM

Shay10825

Oct2-04, 12:42 AM

Ivan Seeking

Oct2-04, 01:16 AM

I have the .25 W 10 ohm resistor, copper wire, electrical tape,
and the D cell battery. No I have not conected the resistor to the battery becase I don't really know how I would do this. Can I touch the resistor when it is connected to the battery?

No I don't have access to a voltmeter.

How much money can you spend? We need to talk about your budget: $10, $20, $50, $5000?

You really must find some safety goggles as well. As for the battery, you can feel free to touch things with the understanding that the resistor will get warm, or even hot. If you make a mistake [like if you use the wrong resistor] the resistor could even go fizzzzz..poooof. If they smoke they can really burn so always be careful - really! I had a glass diode vaporize between my fingers once and I still have a scar over twenty years later.

Check the battery frequently. This should not get hot; slightly warm is okay but no more.

You should look up the RC Color code - that is the Resistor Capacitor color code. Learn how to use it. This is a required first step for any resistor work.

Is there any chance of using a voltmeter at school?

Make sure that you understand ohms law. Do you?

Gokul43201 makes a great point. Be thinking about ways to minimize the mass of the blade. We can increase the power but we are limited. The batteries are the real limit here. How many do you think is reasonable? For example, twenty D cells may be a bit cumbersome for a knife; this would allow for a very powerful butterknife though! :tongue2:

Gokul43201

Oct2-04, 01:32 AM

I hate to say this again, but with a knife-holder-heater, there are several advantages :

1. More room for more batteries, so more power. Also more room, in general, for everything else.

2. More room for resistance wire avoids complications of embedding in the blade and the drawbacks of having an external circuit running outside the blade

3. Heater is largely not exposed to air, so lower losses

4. Heater wire can be made to impart heat to only the front (cutting) edge of the blade. Because stainless is such a poor conductor, this will work.

5. You can simply use AC from a wall outlet, and have no lack of power at all. There will be enough power to heat the knife in less than half a minute.

Ivan Seeking

Oct2-04, 01:39 AM

Ohm's law tells you that the voltage measured across your resistor is equal to the current flowing through the resistor multiplied its resistance: Volts = Amps X Ohms. If you know any two values you can calculate the third.

The watts of power produced - in this case the rate that heat energy is produced in the resistor - is calculated by multiplying the voltage measured across the resistor by the value of the current flowing through it: Watts = Volts X Amps. If you know any two the third can be calculated.

It is important that these points are clear. This allows you to start calculating alternative values to try.

If you are producing up to but no more than 1/2 watt of power in the resistor, you need a 1/2 watt resistor. You can also get 1 watt, 5 watt, 10 watt, and up to resistors capable of handling tens of thousands of watts. So, the batteries and the blade become the real issues.

Ivan Seeking

Oct2-04, 01:41 AM

I hate to say this again, but with a knife-holder-heater, there are several advantages :

I think Shay10825 needs to make some engineering decisions here. :biggrin:

Gokul43201

Oct2-04, 01:41 AM

Also, I can't say I really think this is going about things the right way.

Call me a whiner, but I think it's a bad idea to go about buying stuff and trying to build things without a working plan at hand.

Maybe, Ivan, you've got a plan...but does Shay have one or know what your plan is ?

For instance, I'm curious where the heater wire actually runs, physically, with respect to the blade, and how a thermal short is provided.

And I don't think it's a great idea to use a resistor (instead of a resistance wire). That's a point source of heat, while a line source will be much better. A point sorce introduces a time constant and increases inefficiency.

If I'm whining too much, just ask me to shut up. I really don't want to be such a wet blanket, you know. :frown:

Ivan Seeking

Oct2-04, 01:42 AM

Come on Shay the pressure's on. The world is waiting!

Ivan Seeking

Oct2-04, 01:44 AM

I appreciate your point.

Is it fair for us to completely engineer the project for him? A dollar's worth of reistors and batteries are the extent of spending so far.

Gokul43201

Oct2-04, 01:46 AM

Ivan, what say you ?

I don't want to look like someone that just walked in and threw a wrench in the works.

Ivan Seeking

Oct2-04, 01:47 AM

Not at all. I'm glad you're jumping in.

Gokul43201

Oct2-04, 01:55 AM

Okay here's my final word (until shay makes a decision) on the direct knife heating design :

I don't see how you can run an external circuit, have it not interfere with the purpose of a knife, and provide a good thermal short to the front of the blade.

So, the only way I can see to implement this is by making a hollow blade (by bending and grinding stainless sheet metal) and having the heater wire run inside it.

Even then, with the power limitations, the heating time is going to be long.

Gokul43201

Oct2-04, 02:36 AM

Ivan Seeking

Oct2-04, 02:46 AM

Shay, the one unanswered question is the maximum current that you can safely pull from a D cell battery. We really need this number. I was going to check this myself but for the life of me I can't find any resistors around here that are small enough. If Cliff doesn't chime we will get this one way or the other. The web sites that I checked have not been helpful. It appears that you can pull at least one amp [some sites suggested more like 3 amps]. If we assume that you can safely pull 1/2 amp, then using two D cells in series, we have 3 volts X 0.5 Amps = 1.5 watts. Of course we have to use a 2 watt resistor having a value of R = V/I = 3/.5 = 6 ohms.

Again, I don't know how far we can safely push this number.

enigma

Oct2-04, 06:21 AM

If you're dead set on a knife with the heating device inside, would you consider a hybrid design?

Have the heating part located in the knife, but the batteries in a holder? Then complete the circuit with contacts of some sort.

I'm concerned that the knife will be too heavy to use (much less be marketable... ) otherwise.

This is a fun thread. :tongue2:

We don't get too many "help me design this, please" threads. Engineering at its best. :smile:

Shay10825

Oct2-04, 11:23 PM

Besides your decision on the above question, Shay, answer these questions :

1. What's your budget ?

2. Do you have someone that can buy things for you online using a credit card ?

3. When's the deadline ?

4. Can you borrow a digital multimeter from anyone ? A neighbor, perhaps ? A small DMM can be bought for less than 10 bucks.

5. Where are you ? In the US, I hope.

6. This is a stretch, but you wouldn't have any experience with and access to a soldering kit, would you (it's not essential, but very handy) ?

1. I don't want to spend over $50.
2. No my mom won't let me
3. I think the report (not the final report) is due Oct. 15 but it could change since we have been through so many hurricanes and I have not been in school for almost a month. I need to as my teacher as soon as I get back.
4. I could just but one from Home Depot for $10. What do I need it for?
5. Florida
6. I have never used one before. I saw one at home depot that I could get for $14 (we are at home depot a lot because of the hurricanes).

I like the idea of a knife warmer but would that be harder to make? I would have to make a toaster like thing but for butter knifes.

"So, the only way I can see to implement this is by making a hollow blade (by bending and grinding stainless sheet metal) and having the heater wire run inside it." - Gokul43201
That's wht I was going to do. I was also thinking about taking two real butter knifes, cut the handle off, put the wire between them and welding them (or soddering them however you would do it) together. I really wanted to use silver because it would heat up faster but I could not find sheets of silver. I think I'm going to have to use stainless steel.

A resistance wire might be better since the heat will run all through the blade. What would be the best thing to get so the blade would get hot the fastest and not blow up the battery. I want to use as little batteries as I can. I think if I'm using D batteries 2 or 3 would probably be the max amount (I think more than that would be too much because D batteries are big).

I got goggles, a .5 W, 10 ohm resistor, and heat sink paste today.

Thanks everyone soooo much for helping me with this. I was going to ask my physics and chemistry teachers for help but these hurricanes have put us out of school for so long. It really does not have to be perfect.

PS - I'm a she not a he lol. Shay10825-Shayla

Ivan Seeking

Oct3-04, 12:23 AM

PS - I'm a she not a he lol. Shay10825-Shayla

Gees I'm sorry. The 10825 sounded masculine to me. :tongue2:

I got goggles, a .5 W, 10 ohm resistor, and heat sink paste today

Great job! You also have some money and access to a metal shop. I think a volt-ohm-amp meter is not only needed here, it is a great investment. I would also check with Radio Shack if you have one near.

The soldering iron is a little tougher. This is your call but I would save the budget and borrow one if at all possible. The amount of soldering needed should be very limited.

Why don't you put on your goggles and connect your resistor to one battery? See how long it takes to get hot. How hot does it get? When you get your volt meter, learn how to measure the voltage, current, and resistance in the circuit. Try to secure the resistor to a small piece of metal. how does this affect the heating? Also, don't forget to look up that RC color code. It is shown on the back of the resistor pack, most likely. If not, Google it.

As for the resistance wire, the nice thing about resistors is that we don't need much length. I never looked at the numbers for the nichrome wire but we can string resistors together in order to spread the heat out more. This also eliminates [mostly] insulation problems.

For example, if we use two, 1/2 watt, 10 ohm resistors connected in parallel, we can safely [without burning up the resistors] produce 1 watt of power across a total of 5 ohms. If we use two, 1/2 watt, 5 ohm resistors connected in series, we can produce 1 watt of power over 10 ohms. So we have a lot of flexibility by playing with the configuration. We could use four or five resistors if needed.

Still need that battery current number. We will be back. :surprised

Ivan Seeking

Oct3-04, 12:40 AM

One thing that I have to add. When I do things like this I usually worry about function first, then esthetics. Also, I think everyone who has commented makes good points here, and I don't know what your best course of action may be, but as for a functional [not pretty] handle, if you opt for a self contained unit, a modified flashlight would hold the batteries and provide a switch. Of course, we don't know if the switch could handle the current because we don't yet know the current. Anyway, just a thought...

Also, you could have the knife connect to a circuit in a separate holder, or you could put the resistors and batteries in a separate holder and use a heavy [massive to retain heat] blade. In other words, use the resistors to make your "toaster like thing" or the TLT.

Ivan Seeking

Oct3-04, 03:10 AM

Okay, here is some good information on the Energizer bunny. Take the following link to [datasheets], [Energizer e2], then scroll down and click on the name (left column) X95 - for D cells.

http://data.energizer.com/

It looks like we could run up to 1.5 amps but that may be pushing things. After a quick review of the graphs and data, I think one amp looks like the safe limit based on intermittent use. What do you all think?

Cliff_J

Oct3-04, 07:56 AM

Ivan - I agree with the 1 amp , I'd wayyy overestimated the power of a D cell battery as well. I guess that's why I cuss up a storm when the batteries are dead in my flashlights. :smile: Besides, the bulbs the closest I could find to small value resistors in my stash, it'd be a whole lot of 220 ohm resistors in parallel otherwise...so the manf data is definitely close enough for here.

Gokul - Your suggestions are good but possibly a bit advanced for the task. I'd always thought of it as a give a fish vs. teach to fish type thing where the process of figuring out how exceeds the result for learning purposes anyways. And household power sources seem out of place unless a good isolated source of low voltage could be found - maybe an old gaming console?

Shayla - How warm does the knife need to be? The hot water in your house is going to be under 140F in most cases and I'd think that would still be plenty warm. Extremely easy to test by filling a cup and placing the knife in for a minute and then slicing butter. Works good on ice cream scoops for at least one scoop! Then Ivan's suggestions, especially with hooking up the resistor and finding out how hot it gets (all by itself and again with the resistor touching metal) are great next steps. Just be careful not to burn yourself or overheat the thermometer as they can explode too.

Cliff

Ivan Seeking

Oct3-04, 04:39 PM

This is also a good time to figure out just how to attach the resistors to metal. This must be done no matter how you proceed otherwise. Again, you want to maximize the amount of surface area [of the body of the resistor] touching metal directly. The the leads cannot touch the metal. Then apply the heat sinking compound to the body of the resistor and the metal. This paste will not cause a short circuit so don't worry about making a bit of a mess.

Edit: Actually, I think that paste could be conductive or non-conductive. The label should indicate this. If not, let me know exactly what you bought and I'll check.

Ivan Seeking

Oct3-04, 06:49 PM

Shay10825

Oct4-04, 03:14 PM

How exactly would I do something like this? Wuld it be easier than what I'm doing?

Ivan Seeking

Oct4-04, 03:35 PM

My take is that you would have a wire connecting the knife to the batteries. I think that's what Enigma meant.

Or, you could design the holder with little contacts to make the connection between the knife [resistors] and the batteries when the knife is in the holder. This would rely on the mass of the blade and the resistors retaining enough heat to do the deed. You might see if this is practicle by using a blade and hot water as Cliff suggested. Do some butterological testing. :biggrin:

Or, you could make a TLT and use a regular knife blade [with enough mass for heat, as above].

Keep in mind that we have quite a bit more power to work with than discussed earlier. We might go as high as 5 watts or more - 3 or 4 batteries. Your chosen design could mandate the practical limit for batteries. Earliler you thought two was the limit if contained in the handle.

Hey, I heard schools opened in Florida today.

Shay10825

Oct4-04, 06:06 PM

I got a volt-ohm-amp meter, and the 10 ohm .5 V resistor yesterday.
Would the wire connectors be too big to fin inside the blade?

I'm going to connect the resistors (the .25 V and the .5 V but not at the same time) to one D cell battery. When I do this could I just tape the ends of the resistor to the ends of the battery? Can I touch ALL parts of the resistor? Can ALL parts of the resistor touch the metal so I can see how hot the blade gets? Can I find out how they get without using a thermometer? Would the resistor and/or battery burn (maybe not burn but feel really hot) before it explodes? If the resistor and/or battery exploded what is the worst thing that could happen? Should I measure the current when I'm testing this out?

I'm sorry I have so many questions.

Yeah I started school today :smile: . The person over the science fair at my school is there on even days and today was an odd day so I'm going to ask her tomorrow about the deadline.

Cliff_J

Oct4-04, 06:33 PM

Yes, you could just tape the ends to the battery.

You could touch all parts of the resistor, but it may get hot.

All the non-metal parts of the resistor can touch the blade. The metal wires should be covered in electrical tape.

If it gets hot enough to melt the butter, that would be a success. So you don't need an exact number. But it would be nice to have a number or comparison (cut as well as knife in hot tap water) for the report wouldn't it?

Yes, the battery will get very warm before exploding. If exploded corrosive stuff is released and might spray out. I'm not sure if it has enough power to self-destruct, but if the battery is hot disconnect it!

You should measure enough to write a good report and answer any questions that fit the hypothesis and what not that you started with. More numbers means more graphs and charts, that usually impresses anyone. :smile:

Cliff

Ivan Seeking

Oct4-04, 06:34 PM

[COLOR=Blue]I got a volt-ohm-amp meter, and the 10 ohm .5 V resistor yesterday. do you mean 0.5 watts?

Would the wire connectors be too big to fin inside the blade?

I don't know how you plan to make your blade.

I'm going to connect the resistors (the .25 V [watt]and the .5 V [watt] but not at the same time) to one D cell battery. When I do this could I just tape the ends of the resistor to the ends of the battery?

That will probably work

Can I touch ALL parts of the resistor?

Yes but, the resistor could get quite hot. Always assume that the resistor will burn. Be careful.

Can ALL parts of the resistor touch the metal so I can see how hot the blade gets?

Not the metal leads. These must touch only the battery; one on each end. You can use black tape to help insulate the leads. If they both touch the same piece of metal [or each other] then you have a short circuit and the battery could overheat. Keep a very close eye on this. Wear your goggles, always. Check the battery frequently to make sure its not getting hot.

Can I find out how [hot] they get without using a thermometer?

That's pretty tough. Getting a feel for things is good for now.

Would the resistor and/or battery burn (maybe not burn but feel really hot) before it explodes?

the resistor would just fizzle and start to smoke. The battery is the thing to watch. You should be fine as long as you don't create a short circuit or use the wrong configuration of resistors. The battery will get hot if there is a problem, but a short circuit could cause the battery to get hot really quickly. If you see or smell smoke, pull the wire loose.

If the resistor and/or battery exploded what is the worst thing that could happen? Should I measure the current when I'm testing this out?

Again, the batteries are the thing to watch. Monitoring the current is a real good idea. In fact, what we should do is to get a fuse in the circuit. You should have an extra fuse with your meter. What is the value? Do you have any one amp fuses around; maybe automotive fuses? I'm sorry I didn't think of this earlier. I think you could still try some quick tests as long as you carefully avoid short circuits.

I'm sorry I have so many questions.

At this point I would be disappointed and worried if you didn't. :biggrin:

Is there an adult around. You should not be doing this without someone around. Make sure that they know what you're doing.

Shay10825

Oct4-04, 08:22 PM

How long do I have to wait for the resistor to get hot? I'm using the .25 W 10 ohm resistor and a D cell battery. I connected it but it did not get hot.

Gokul43201

Oct4-04, 08:26 PM

About touching the resistor : My calculations suggest that the resistor won't get hot for at least a few tens of seconds...perhaps longer. I suggest you touch the body of the resistor once every 5 seconds or so, with the tip of one finger. After this test, I would try again (giving the battery a little time to cool down) with the two resistors connected in parallel.

After this, you could try a test with the resistors enclosed in a piece of aluminum foil - which simulates the blade - and see how hot the foil gets. You could even have a piece of butter sitting on the foil.

Remember, you don't want the foil to touch any of the metal wiring or the battery terminals. So, you can do one of two things : (i) cover all the wiring in tape, or (ii) wrap it all in a sheet of paper (don't do multiple wraps; two at most) and then wrap this in foil. Using the heat sink paste will be useful.

Shay10825

Oct4-04, 08:30 PM

So I let the metal touch the whole resistor that's covered in tape (the wires too) or should I only let the metal touch the part of the resistor with the different color lines?

Gokul43201

Oct4-04, 08:32 PM

Shay10825

Oct4-04, 08:37 PM

I'm thinking (since Ivan says that 1 amp is safe) that you probably want a 2 ohm, 1 watt (higher that 1 watt, if possible) resistor.

That will draw about 0.75 amps and dissipate a healthy 1.1 watts. Much better than the measly 0.2 watts your getting with a single resistor. With your two existing resistors, in parallel, you will get about 0.4 watts.

The resistor at Radio Shack with the lowest resistance (in ohms) is a 10 ohm resistor. They also only have .25 and .5 watt resistors. How can I use a 10 ohm resistor with .25 or .5 ohms?

Gokul43201

Oct4-04, 08:49 PM

Here's an idea : Use 5 (or even 7) of your 10 ohm, 0.5 watt (0.25 watt resistors would work too) resistors in parallel. This achieves :

1) Your effective resistance is now 10/5 = 2 ohms, which gives you better than 1 watt without exceeding the power capacity of any of the individual resistors,

2) It provides a means of having a line (distributed) source of heat instead of a point source. See the drawing below. I'm fixing it now...gimme a couple minutes...

/\ blade
/ \
/ \
/ \
/ \
/ \
/ \
/ \
| Resistor (R) |
| |------VVVVV------| |
| | R | |
| |------VVVVV------| |
| | R | |
| |------VVVVV------| |
| | R | |
| |------VVVVV------| |
| | R | |
| |------VVVVV------| |
\ | | /
| |
| |
-----Battery-------

Shay10825

Oct4-04, 08:52 PM

So the resistors would be connected together and then connected to only one end of the battery?

How do you know "resistance is now 10/5 = 2 ohms"? Is it a rule that if you have resistors parallel then the resistance is the resistance of each 1 divided by the number of resistors you have?

Gokul43201

Oct4-04, 08:56 PM

No, hang on....let me fix the picture. Gimme a minute here.

Okay, I'm done now.

Shay10825

Oct4-04, 09:02 PM

Would I use one long wire to connect all of this or 2 different wires? I took a speaker wire and cut off the plastic covering it. I'm using the copper wire inside. Would I only use one strand or double them up to make it thicker?

Gokul43201

Oct4-04, 09:07 PM

How do you know "resistance is now 10/5 = 2 ohms"? Is it a rule that if you have resistors parallel then the resistance is the resistance of each 1 divided by the number of resistors you have?

Yes, the effective resistance of a bunch of parallel resistors is R/n where R is the value of each resistance and n is the number of resistors.

The formula for the effective resistance of resistors in parallel is :

\frac {1}{R_{eff}} = \frac{1}{R1} + \frac{1}{R2} + \frac{1}{R3} + ...

If R1=R2=R3=..., let's call them all simply R, then of course :

\frac {1}{R_{eff}} = \frac{1}{R} + \frac{1}{R} + \frac{1}{R} + ... = \frac {n}{R}

Inverting this, gives you the required result.

Gokul43201

Oct4-04, 09:10 PM

Would I use one long wire to connect all of this or 2 or 3 different wires? I took a speaker wire and cut off the plastic covering it. I'm using the copper wire inside. Would I only use one strand or double them up to make it thicker?

I'll answer this question in a bit. But first ...how did the test go ? Did the resistor get hot at all ?

Now about your question : You could simply use 2 long wires, one on each side. However, to carry about 1 amp of current you need about 22 gauge wire or thicker (20 gauge would be safer). See this table, and look at the first and last columns. http://www.powerstream.com/Wire_Size.htm

Now if you look at your original wire, it will say on the insulation, what the gauge is (something like 18 AWG or 20 AWG - "AWG" stands for American Wire Gauge). If that number is smaller than 22 AWG, then you are good. Remember, the smaller the gauge, the thicker the wire.

What gauge does it say it is ?

Gokul43201

Oct4-04, 09:23 PM

One more thing : If you are going to use more of this wire, it is better to remove the insulation only where you are making connections (like at the ends). Don't remove all of the insulation from the wire.

Ask, if you don't know how to do this. There's a couple of correct ways to do this.

Shay10825

Oct4-04, 09:29 PM

I got the wire form Home Depot so I don't have what it came in (they cut how much you wanted off for you) so I don't know if it is a 20 gauge or not. The wire is about 2 mm thick (with the insulation) so it would be about a 12 gauge? Should I get one that is 20 gauge tomorrow just to be safe?

What is the correct way to remove the insulation?

Gokul43201

Oct4-04, 09:36 PM

How much wire did you buy ? You didn't remove the insulation off all of it, did you ?

Look carefully at the insulation again...the gauge will be printed on it, in tiny letters...probably along with manufacturer and safety information.

Gokul43201

Oct4-04, 09:41 PM

What is the correct way to remove the insulation?

I'm guessing you don't have a wire stripper. I'm also guessing the wire is not solid - there's several thin strands of copper wire bunched together, right ?

Use a razor blade or utility knfe/box-cutters. Be very careful. Carefully cut away about 1/2 inch to 1" off of the ends, making sure you don't cut the copper wires themselves. Apply gentle pressure with the blade as you rotate the wire.

Shay10825

Oct4-04, 09:46 PM

I bought only 1 ft of it (it was only .17 cents) . No. The wire was not just a single wire it had two insulated wires connected (parallel) to each other so I cut down the middle of it to have on wire. Then I took the insulation off one (out of the two now) of the wires. There are red dots on it but it does not look like it spells out anything. It did not have any manufacturer and safety information. I could just but more but in a pack that says the gauge.

Shay10825

Oct4-04, 09:48 PM

there's several thin strands of copper wire bunched together, right ?

Yeah there is. I took pliers and ripped the insulation off.

Would it be better if I got something that was just one wire?

Gokul43201

Oct4-04, 09:55 PM

That information would have been on the outer insulation...the one you slit down the middle. Did you throw that away ?

Gokul43201

Oct4-04, 10:00 PM

Yeah there is. I took pliers and ripped the insulation off.

Would it be better if I got something that was just one wire?

The plier technique is not the best way. Follow the razor blade approach. After cutting around the insulation, you should be able to simply pull it off.

The solid wire will be a lot less flexible than multi-wire. Just be careful not to cut through the wires.

The 1 ft of wire you have may be enough...let's wait and see. You will use a bunch of it for testing, but you can always buy more, if you need to, right ?

Psssst, Any luck with the battery test ???? :confused:

Shay10825

Oct4-04, 10:07 PM

Yeah I threw it away. Yeah I can buy more. Should I still do the battery test with the copper wire I have and if so should I only use one strand or the number of strands that were in the wire?

Gokul43201

Oct4-04, 10:18 PM

Use all the strands. And yes, do the test with what you have. Right now, your current will be small, so the wire is most probably just fine.

How do you connect the wire to the battery terminals and the resistor leads ?

Gokul43201

Oct4-04, 10:24 PM

So I let the metal touch the whole resistor that's covered in tape (the wires too) or should I only let the metal touch the part of the resistor with the different color lines?

Do the basic test before trying aluminum foil.

The foil should not touch any of the metallic parts of the circuit - the resistor leads, the wire, the battery terminal. If the leads and wires are all taped up, it's okay for the foil to cover them too. To get a more realistic idea, I would double up the foil, to make it twice thick.

And use the heat sink paste over the ceramic part of the resistor (the part with the color code), and make sure the foil is making contact with the paste, or even the ceramic itself.

Shay10825

Oct4-04, 10:25 PM

I tape the wire to the battery and wrap it around the resistor wires.

Gokul43201

Oct4-04, 10:28 PM

I tape the wire to the battery and wrap it around the resistor wires.

Okay, just to make sure the wire is making good contact to the battery terminals, you might want to press down with your fingers. Make sure you only put your finger over the tape, and not the metal. Not that it will harm you...

Ivan Seeking

Oct4-04, 10:53 PM

I'm thinking (since Ivan says that 1 amp is safe)

Keeping in mind that this was for the Energizer e2 and intermittent use only.

Gokul43201

Oct4-04, 10:58 PM

Yay, you're here !

Hope that didn't sound like I was going to point my finger at you if something went wrong ?

Okay, I've independently verified that the Eveready A95 (D-cell) can be used with 1.5 ohms (drawing 1 amp) for 4mins out of every 15 mintes, and will have a life of 15 hours of usage at this current.

So I too would say that 1 amp draw should be okay, as long as you don't run it for more than 4 minutes at a time, with at least 10 minute breaks.

Ivan Seeking

Oct4-04, 11:20 PM

I think you're doing a great job! I just wanted to add a word of caution.

Shay10825

Oct5-04, 06:17 PM

I probably can't experiment and post anything tonight because I have a really important Physics test to study for (what I get on this determines my grade :frown: ), and a ton of Physics homework. I might be able to start back testing tomorrow.

Shay10825

Oct9-04, 10:28 PM

I just tried connecting the resistor to the battery and it does not get hot. I wrapped the copper wire around the resistor wire then taped it to the end of the battery. I did this for both sides of the resistor and battery. Why is it not getting hot? Could it have something to do with the way I'm wrapping the copper around the resistor?

Shay10825

Oct10-04, 01:06 PM

And use the heat sink paste over the ceramic part of the resistor (the part with the color code), and make sure the foil is making contact with the paste, or even the ceramic itself.

I did not use the heat sink paste over the ceramic part of the resistor. Could this be the reason why the resistor does not get hot?

Ivan Seeking

Oct10-04, 03:47 PM

First, which resistor are you using - the 1/4 watt or 1/2 watt resistor? Ten ohms right?

The heat sink paste only serves to transfer the heat from the resistor to the metal.

Do you understand how to measure voltage and current?

You should measure about 150 milliamps of current flowing with the circuit made Check that you have current flow. Be patient. We are starting slowly but things will pick up quickly once we're all set in just a few more steps.

Ivan Seeking

Oct10-04, 04:17 PM

Did you find out what the deadline is for your report and project?

How did you do on your physics test? :biggrin:

Shay10825

Oct11-04, 02:17 PM

First, which resistor are you using - the 1/4 watt or 1/2 watt resistor? Ten ohms right?

Do you understand how to measure voltage and current?

Both the .25 and .5 watt to see which works best.

Do I understand how to measure voltage and current with the multimeter? No. What should the wires and needles be touching?

Did you find out what the deadline is for your report and project?

How did you do on your physics test?

Yeah the report and the project is due November 18.

Not so good :cry:. I got a 72 :frown: . But it does not determine my grade anymore because the school board extended the quarter. So we will have 1 or 2 more tests.

Ivan Seeking

Oct11-04, 07:02 PM

Read this
http://www.ehow.com/how_16767_voltmeter.html
the section about measuring with a voltmeter here
http://www.facstaff.bucknell.edu/mastascu/eLessonsHTML/Basic/Basic3v.html#Measuring
and pages 1,2 and 6, here
http://hibp.ecse.rpi.edu/~connor/education/IEE/labs/Lab_1_Current_Voltage_and_Resistance.pdf

There should also be some instructions with the meter. Make sure that you read those as well.

Do you have a Radio Shack nearby?

Not so good :cry:. I got a 72 :frown:

:frown: ...better next time. :smile:

Shay10825

Oct11-04, 07:53 PM

Do you have a Radio Shack nearby?

Yeah there is a radio shack nearby. why?

Ivan Seeking

Oct11-04, 08:20 PM

They have some really nice, very short books that introduce the basics with pictures for clarity. They are small pamphlets - thin paperbacks and hand written! You will usually find them in the electronic's sections with the resistors and transistors. It would be worth a stop to take a look and see what they have. In fact, there is a thin, full sized (8.5" X 11") book, green I think, that covers much of the basics.... In fact I just checked and I have my 1983 version. It is called "Getting Started in Electronics"; Cat no. 62-5003. It is a handy, practical reference.

Gokul43201

Oct12-04, 07:25 PM

Shay, with just one 10-ohm resistor, your not generating enough power to get it hot. You will need either a higher power, lower resistance resistor, or a bunch (say 5 to 7) of .5 watt, 10-ohm resistors to generate enough heat.

You may also not be making proper contact between wires. The right way to maximize contact (short of soldering or using wire nuts) is to hold the wires side-by-side and twist them together, several times. Then tape over this, tightly.

To check if your circuit is okay, test it with the multimeter. First connect the probes of the meter to the socets corresponding to voltage measurement. The black (negative) lead goes into the ground socket, and the red (positive) lead goes into the socket labeled V (not the one labeled I, for current).

Now the dial on the meter has different settings. Find the positions for DC voltage measurement, and set it to the value that is just above the expected voltage of 1.5V (so, if there's a 5V or 10V position, use that). Check the voltage across the battery and across the resistor. They should be nearly the same. Similarly, by changing the dial position to read resistance (ohms), you can check the resistance across one or more resistors. The current can be found using Ohm's Law.

Shay10825

Oct12-04, 10:41 PM

When I'm titling this invention for the science fair does it need to be a question?

Ivan Seeking

Oct13-04, 09:34 AM

Did the rules for the fair indicate this?

Shay10825

Oct15-04, 12:07 PM

Yeah never mind I found it. It has to be something catchy. Any ideas :smile: ?

Cliff_J

Oct15-04, 12:15 PM

You might borrow from the old "...building a better mousetrap..."

Maybe two words or word fragments like many other products so maybe a combo of thermo and knife or slicer or something

Or visit places that sponsor inventions like Hammacher Schlemmer, they have tons of gadgets.
http://www.hammacher.com/
http://www.hammacher.com/sfi/sfimain.asp

Hope all is progressing well.

Cliff

Shay10825

Oct15-04, 02:10 PM

The Better Butter Cutter (BBC) :biggrin:

Cliff_J

Oct15-04, 04:07 PM

Hey I own a "Butter Butler", its handy for the few times I use butter. My dad has a coffee mug with a adapter to plug in the cigarette lighter to keep the coffee warm while driving, also handy. Who knows what people might like?

http://www.krbctv.com/Does%20It%20Work/DIW%20Archive/diw_archive_cooking_butter%20butler.htm

Ivan Seeking

Oct20-04, 03:02 AM

Shay,
I wanted to try this before saying too much. Once you know how to use your meter you are safe to go ahead as follows: Using your 1/2 watt resistor and two D-Cell batteries connected in series [end to end like in a flashlight], go ahead and apply 3 volts across the resistor for ten or fifteen seconds. The resistor will get hot. If you leave the circuit connected long enough, eventually the resistor should burn up.

Potential [V]= 1.5 volts + 1.5 volts = 3 volts
Resistance [R]= 10 ohms
Current [I]= 3 Volts / 10 ohms = 0.3 Amps
Power [P]= VI = 3 volts X 0.3 Amps = 0.9 watts

Since the resistor is rated for 0.5 watts and you are running it at 0.9 watts, you are running the resistor beyond its design limit. This is why it starts to get too hot.

Next, use 4 D-Cells, a 1.0 watt -10 ohm - resistor [available at Radio Shack], a piece of metal about the size of a knife blade, and your heat sinking compound.

By running the batteries in series we get:

1.5 volts X 4 = 6 volts
I = 6/10 = 0.6 amps
P = 6 volts X 0.6 amps = 3.6 watts

Again, I used a 1.0 watt resistor, so I was running the resistor at 360% of the design limit.

I did this using a fairly heavy piece of brass sheet metal - about 1/2" X 2" in size. First, a good layer of heat sinking compound was applied to the metal. Then a toothpaste sized blob was applied and the resistor was secured to the metal using only the cohesiveness of the compound. Pull the resistor's leads back so they don't short out on the metal and wiggle the resistor around a little to ensure a good bond. Also make sure that it's buried in the heat sink blob. This will pull heat away from the resistor and transfer it to the metal; thus preventing the resistor from burning up.

After applying power I that found that an area of about 1/2" X 1/2" got hot very quickly - in less than 30 seconds. Afer five minutes the entire piece of brass was very warm, with half of it too hot to hold. The batteries did not get hot and the resistor was hot but it seemed fine. So it appears that we can safely heat sink the resistors and overdrive them. This seems to work pretty well.

Arctic Fox

Oct25-04, 06:12 PM

You may not have seen the original thread in GD where some similar suggestions were made. Shay10825 is hooked on a heated blade for now.

Hooked, huh?

No chance in a heated wire on a cheese cutter?

http://www.twinsupply.com/fdick_cutlery/81055-00.jpg

Gokul43201

Oct26-04, 12:15 AM

After applying power I that found that an area of about 1/2" X 1/2" got hot very quickly - in less than 30 seconds. Afer five minutes the entire piece of brass was very warm, with half of it too hot to hold. The batteries did not get hot and the resistor was hot but it seemed fine. So it appears that we can safely heat sink the resistors and overdrive them. This seems to work pretty well.

There, now if you can only convince Ivan to ship over his stuff to you. :wink:

I'm not sure how long the resistors will last if you overload them repeatedly. You could consider using a couple or more resistors in parallel, to divide the power.

Ivan Seeking

Oct26-04, 02:03 AM

The advantage is the significantly reduced mass and volume of the resistor. To a point she should be okay but I agree that this is unconventional.

Is this just our project now or is Shay coming back? :biggrin:

Gokul43201

Oct28-04, 10:39 AM

The advantage is the significantly reduced mass and volume of the resistor.

In my approximation, the mass of the blade dominates the mass of the resistor.

Shay's got a couple weeks to go and seems to have fallen prey to the powerful yet illusory bliss afforded by procrastination. Haven't we all bitten on that, sometime or the other ?

Shay10825

Oct29-04, 05:55 PM

Ok I'm back. I'm sorry. I've just been so busy with a ton of homework and tests these last couple weeks. It's been crazy at my school. The 9-weeks just ended so I'm working all weekend on this project. Ivan Seeking I'm going to try what you said right now.

Sould I use a 20 gauge wire???

Shay10825

Oct29-04, 07:36 PM

Would this work?
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=270-396

or do the batteries have to be on top of each other?

Shay10825

Oct29-04, 07:43 PM

When I connected the .5 watt 10 ohm to the resistor it seemed hot when I touched it. It was weird (or maybe I'm just weird and it's suppose to feel like this). Are the lines on the resistor the only thing that's suppose to get hot? It did not get the metal hot. Is this because I did not put and heat sink on the metal? Is it suppose to be like this?

Ivan Seeking

Oct30-04, 07:07 PM

20 gauge is perfect.

If you look at the connections on the battery case that you linked, you will see that they are wired in series - as if end to end, or on top of each other. So this should work fine. This means that you will have about 6 volts to work with = 4 X 1.5 volts per battery.

On your last question I'm a little confused. The entire body of the resistor will get hot. The lines are your color code - which you are supposed to know by now. :mad:

How many batteries are you using? Two? Then yes, the resistor should get hot. Again, this is producing 0.9 watts of heat in a 0.5 watt resistor. In principle, the resistor should eventually burn up or fail from excessive heat.

When you talk about the metal, do you mean the leads on the resistor or did you get a separate piece of metal? The metal leads on the resistor cannot touch your metal knife blade; or any other conductor for that matter. And you will need to use the heat sink compound to transfer the heat from the resistor's body to the knife blade. This compound will not short out the resistor leads; so it can touch the leads.

Make sure that you learn how to measure the voltage and current as you do this. check that the values measured agree with what you calculate.

Shay10825

Oct30-04, 07:11 PM

What would happen if I used a 22 gague wire?

Shay10825

Oct30-04, 07:19 PM

Which one is better?

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F010%5F006%5F000&product%5Fid=278%2D1388

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F010%5F003%5F000&product%5Fid=278%2D1222

Cliff_J

Oct30-04, 07:24 PM

Home Depot. :smile:

They have speaker wire for a few cents a foot, one dollar would get more than you need.

Glad to hear its getting warm now, all that's needed is some compound to get the heat to the blade and then test to see how well it works and tweak from there.

Cliff

Shay10825

Oct30-04, 07:34 PM

Does it matter if I use a 2-conductor Speaker Wire or a Hookup Wire? What happens if it is a 22 gague wire?

Ivan Seeking

Oct31-04, 01:52 AM

22 gauge is fine but don't go any smaller. If the wire gets too small it could start to overheat and melt the insulation. The exact ampacity of the wire depends on the insulation used, but 22 gauge should be fine. I think the lowest rating for this size wire is a little over 2 amps.

karmicthreat

Oct31-04, 03:22 PM

Man now I'm hooked on butter cutter technology. Instead of using a resistive circuit to heat the knife why not use a LC circuit resonating inductively with the metal in the blade to heat it. You would get nearly instant heat in the blade I'm not sure on the efficentcy though. This is the same method that inductive ranges and the metals industry use.

Also since you could get nearly instant heating on the blade, divide the handle into two electrically insulated halves. have a circuit measure the capacitance between the two hallves, when someone is holding it there will be a fairly reasonable range of change and the knife can activate. This way you won't wear down the battery of the knife by having to wait for it to get up to temp. Or having to dump a bunch of extra power into the knife to get it up to temp fast enough.

This sounds nearly as weird as the electric spoon I helped design. Had a temp sensor in the laddle and charged its self inductively when in its cradle.

Shay10825

Nov7-04, 02:24 PM

I'm so happy :smile: !! The whole piece of metal got hot in a very short amount of time. I used aluminum. The problem is getting the resistor between the two pieces of metal and connecting the two pieces of metal together. A guy at Home Depot told me to use some stuff called liquid nails. Would this work?
http://www.homedepot.com/prel80/HDUS/EN_US/diy_main/pg_diy.jsp?CNTTYPE=PROD_META&CNTKEY=misc%2fsearchResults.jsp&BV_SessionID=@@@@1241869940.1099859104@@@@&BV_EngineID=ccdkadcmmlfdfeecgelceffdfgidgjj.0&MID=9876

I also bought wire connectors. They are the kind where you put the wires inside it (it is a tube) and you use pliers to push the middle of the tube in. These will have to be inside the blade. Is there any other thing that I could use to connect the wires because if this is in the blade the blade will be too thick.
http://www.homedepot.com/prel80/HDUS/EN_US/diy_main/pg_diy.jsp?CNTTYPE=PROD_META&CNTKEY=misc%2fsearchResults.jsp&BV_SessionID=@@@@1241869940.1099859104@@@@&BV_EngineID=ccdkadcmmlfdfeecgelceffdfgidgjj.0&MID=9876

Could I use heat shrink tubing to cover the blades of the resistor? I would put the blade inside this tube and put a fire in the tube to heat it so it will shrink around the blade? Can I do this? Or will the fire hurt the resistor?
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=278-1627

Cliff_J

Nov8-04, 10:54 AM

Glad to hear its getting very warm now. Of course you could use the multimeter and other resistors and battery combinations to get different heating...wink wink...

Liquid nails would work ok (it comes in different styles for adhering different materials) but just plain 100% pure clear silicone in a squeeze tube would work ok too. I saw some liquid nails brand silicone in a squeeze tube a Walmart just recently for like $3 and you don't need a caulking gun to apply. (its probably at Home Depot too) Clear looks cool. :smile: Oh, and work in a well ventilated area and maybe open a window, all this stuff stinks and you shouldn't breathe more vapor than you need.

Instead of wire connectors, you could likely just twist the wire and the resistor lead together really well. Normally solder would then be used to keep the twist together and improve the connection but if you twisted tight and then used heat-shrink tubing that should work well enough.

The heat-shrink is sometimes tougher to get to shrink evenly than one would think beforehand. For a very short period of time the fire wouldn't affect the resistor but it wouldn't take too long to damage it and heatshrink can cause that. If you could shrink the heatshrink outside the knife before assembly that would ease the process tremendously.

Good old electrical tape works as well (but doesn't look at cool) and if you buy any color other than black it doesn't leave the same gooey adhesive residue when you remove it (if you need to). You can get 3M brand colored electrical tape at Home Depot for like $3-4 a roll.

Oh and twisting wires and taping the connection is a bad practice for any kind of long-term application, but for this that shouldn't be an issue at all....

Cliff

Shay10825

Nov9-04, 07:23 PM

When I cut the sheet metal the edges are sharp. How can I make it so they are not sharp?

Gokul43201

Nov9-04, 08:25 PM

You can de-burr the edges of the sheet metal using a belt/wheel grinder, but if you dont have access to either, just use a file to blunt the edges. If you don't have a file, sand-paper will work.

How thick is your sheet metal ?

Shay10825

Nov9-04, 08:39 PM

How thick is your sheet metal ?

It's 1 mm thick

BoulderHead

Nov9-04, 08:51 PM

Has anyone discussed heating only a slender wire and using it to slice through the hard butter?

Of course, for the really frozen hard butter more drastic steps are demanded;
http://history1900s.about.com/library/graphics/viet48.jpg
...it does toast, also. :tongue:

Ivan Seeking

Nov9-04, 11:15 PM

Shay, please note the comments made by Cliff and Gokul. We are running the resistor [10 ohm, 1 watt, using four batteries in series] at 360% of the design limit of the resistor; or 3.6 watts. We get some forgiveness [I think quite a bit, maybe even enough] by heat sinking the resistor to the knife blade, but how much forgiveness we get is a total guess. There is a danger that the resistor will fail due to excessive heat and after some number of cycles. In principle, if we had no heat sinking you would need to use four of these resistors - two resistors in series, and these in parallel with another two in series. This would produce the same amount of heat energy but over four resistors instead of one - putting us within the resistor's rated value of one watt each.

Normally one would never design a circuit or device without knowing exactly how long each part should last. One never designs a device to exceed the rated values of the parts used. The only way to know how long the one resistor configuration will last is to run some tests and make it fail. Otherwise you are taking a pretty serious risk of having a failure at an inopportune moment. You may want to consider using four resistors instead, but this is a judgment call on your part.

Ivan Seeking

Nov9-04, 11:36 PM

How hard will it be to replace the resistor if it fails? You need to consider your butter knife maintenance needs as well. :biggrin:

Gokul43201

Nov10-04, 10:49 AM

Boulder just loves those fire-spitting toys, doesn't he ? :wink:

Ivan Seeking

Nov10-04, 05:25 PM

Boulder just loves those fire-spitting toys, doesn't he ? :wink:

I think dinner at Boulderhead's house could be quite dangerous. :surprised

Gokul43201

Nov11-04, 10:14 PM

Tiny detail : a hair dryer is ideal for heat shrink tubing...no need to use Boulder's flamethrower.

Boulder, there are problems with having an uninsulated exposed circuit element...but with a (5-10 mil) coating, that would probably work very well.

I think this is a good idea. The only problem now would be getting the right material for the wire, and buying it without using plastic.

Shay10825

Nov13-04, 09:32 AM

When I put the resistors on a piece of metal then but another piece of metal on top there is a big space between the two pieces of metal because of the resistors. How can I make this space small? If I just put liquid nails in the space the knife will be too big. What can I do?

Also is there some kind of rechargeable battery that is smaller than a D cell battery but will work just like one??

Ivan Seeking

Nov13-04, 06:43 PM

Why do you want to fill the space? If the resistor makes good contact with one metal surface - the front surface - wouldn't that work?

As for battery size, you may be able to use a rechargeable C...I will check a little later.

Shay10825

Nov13-04, 09:23 PM

Why do you want to fill the space? If the resistor makes good contact with one metal surface - the front surface - wouldn't that work?

Are you saying that I should put the resistor on one piece of metal then cover the resistor (which would be on the back of the metal blade)?? Or should I put the resistor in between two pieces of metal? If I use one piece of metal what should I cover the resistor with?

Ivan Seeking

Nov14-04, 03:42 PM

hmmm, what do you think? On one hand it seems easier to use only one piece of metal. On the other hand, you need to cover the resistor with something... another piece of metal would make sense. I guess my thinking is this: You really only need one heated surface. If the other surface is metal or something else, would it matter? You might also consider that the resistor might fail and need to be replaced.

This is part of the creative design process and I don't know the best answer. You're the engineer. It sounds like you need to think this through. I would tend to use one heated surface and then attach something on the back for protection [of the resistor and heat sink compound] that can be removed if needed. It does need to be a good cover since you certainly don't want heatsink compound or a resistor on your toast! You need to be able to clean the knife...I'm not sure. Maybe using silicone to glue another piece of metal would work; as I think Cliff suggested earlier.

Ivan Seeking

Nov14-04, 04:11 PM

Okay, it looks like you can use four rechargeable C cells.
http://data.energizer.com/PDFs/nh35.pdf

According to the data sheet on the battery [above] you can expect a little less than 5 knife-hours per charge if using an operating current of 500 mA = one half amp. These cells have a slightly lower output potential of 1.2 Volts [remember that the other batteries are 1.5 volts], so this will reduce the current and power a bit. Keep in mind also that these numbers may vary a bit between brands.

[1.2 Volts/battery] X [4 batteries] = 4.8 volts
V = I X R so I = V/R = 4.8 volts / 10 ohms = 0.48 Amps [about a half amp]
Power = V X I = 4.8 volts X 0.48 amps = 2.3 Watts of total heat energy

We have reduced the voltage by 20%:
1.5 volts - 1.2 volts = 0.3 volts, and [0.3 volts / 1.5 volts] = 0.2 ---> 20% reduction

But the power is reduced by:
3.6 watts - 2.3 watts = 1.3 watts, and 1.3 / 3.6 = 0.36 --> 36% reduction

Ivan Seeking

Nov14-04, 05:36 PM

Ivan Seeking

Nov14-04, 06:03 PM

okay I'm a little confused but the datasheet for size AA rechargeable batteries says that these would work just as well. In fact the D, C, and AA cells of these type all claim 2500 mAH's to one volt. So, it looks like you can use AA's. You should pay a price of time for size, ie. bigger batteries last longer, but for some reason this is not indicated as being true. In either event, smaller batteries mean less time between charges, normally. Even if the data sheets are in error you should still expect a couple of hours of knife-time between charges.

Edit: I see what's happening ;this should be good information. It turns out that you pay such a high energy storage price for using rechargeable batteries that the performance varies little between sizes. In other words, a rechargeable size D has about the same capacity as a non-rechargeable size AA. Also, since they have about the same internal resistance as the larger batteries, the maximum current is about the same.

Go here http://data.energizer.com/ and select datasheets and compare the listed capacities for each type.

Shay10825

Nov15-04, 07:08 PM

I just bought an 8 AA battery holder. How can I make it so it will work with 4 AA batteries?? The 8 AA battery holder works better for a handle than the 4 AA holder because it is longer. Also with the 8 AA holder I will be able to disconnect the blade from the holder. If there is no way I can use 4 AA batteries in the 8 AA holder then how many resistors would I need for 8 AA batteries?? Would I need 2 10 ohm 1 watt resistors?
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=270-407

Also how do I make an on-off switch??

Ivan Seeking

Nov15-04, 08:05 PM

It looks to me like you could use only one side. You could just connect to the spring located after the four batteries, in the empty slot, or you could short circuit the spring to the output contact where the other four batteries would be. As long as your four batteries are in series [end to end, i.e + to - to +...] it should work.

If you wish to use all eight batteries then you should go through the calculations to determine the resistance needed. We can check your results. How much power do you wish to risk across the resistor[s].

I really liked the reduced power of 2.3 watts, rather than 3.6 watts, on the 1.0 watt resistor. You don't want the resistor to fail. Remember, we are guessing here...a very, very dangerous thing to do. We are absolutely safe at 1.0 watt per resistor.

Shay10825

Nov15-04, 08:44 PM

Could I use 8 AA batteries and one 100 ohm 1 watt resistor? Then I would only use 1.44 watts out of my 1 watt limit.

8(1.5) = 12 volts
12 volts/ 100 ohms = .12 amps
12 volts * .12 amps = 1.44 watts
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=271-152

Ivan Seeking

Nov15-04, 08:58 PM

That looks real good! One minor correction is the voltage: Remember the rechargeable batteries are rated at 1.2 volts, instead of 1.5 volts.

If you want more power you can add a second or even a third resistor in parallel. This means that the resistors are not in-line, like the batteries, but they are side by side, so to speak. Note that in a parallel circuit, the resistance in each leg is still 100 ohms. If you use two 100 ohm resistors in parallel, the power is doubled. This is not true with two resistors in series though since the resistance goes up and reduces the current.

Series circuit
+ ---- R1----- R2 ----- -

Parallel circuit
+ ---- R1 ----- -
+ ---- R2 ----- -

Shay10825

Nov15-04, 09:05 PM

Could I use the 1.5 volt battery and just use one resistor?

Ivan Seeking

Nov15-04, 09:09 PM

One more clarification. Note that the current demand on the batteries has dropped significantly as a result of increasing the voltage and resistance. This is why we can start adding resistors in parallel, if needed.

Ivan Seeking

Nov15-04, 09:15 PM

Could I use the 1.5 volt battery and just use one resistor?

I was only suggesting the other resistors in the event that you want more power. This will depend on how happy you are with the knife's performance. The rechargeable batteries only put out 1.2 volts as a nominal value. If you want to use regular batteries, the 1.5 volt type, then we need to check the datasheets again for AA throw away batteries; just to double check the maximum current. We were basing this all on size AA rechargeable batteries. Do you want me to check the regular AA batteries?

Shay10825

Nov15-04, 09:34 PM

Do you want me to check the regular AA batteries?

Yes could you please :smile:. I want to use the 100 ohm 1 watt resistor and 8 AA batteries.

Ivan Seeking

Nov15-04, 09:56 PM

Okay hang on...

While you're waiting, I'm just curious, what about the 8 X 0.3 volt drop concerns you? This actually puts us within the 1.0 watt limit per resistor at 0.92 watts.

Shay10825

Nov15-04, 09:59 PM

One more thing: Shay you should pick up a fuse block and a [some] 3/4 - 1 amp fuse[s] to put in your circuit. This will prevent any runaway short circuit problems which could be dangerous. You can probably mount the fuse block on the battery holder.

Edit: You can get this at Radio Shack for a few bucks.

Should I get this?:

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=270-742

What kind of fuse should I get?:
http://www.radioshack.com/category.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F004%5F008%5F000&Page=1

http://www.radioshack.com/category.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F004%5F003%5F000&Page=1

http://www.radioshack.com/category.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F004%5F002%5F000&Page=1

http://www.radioshack.com/category.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F004%5F000%5F000&Page=1&find=fuse(keyword)&site=search

Shay10825

Nov15-04, 10:02 PM

While you're waiting, I'm just curious, what about the 8 X 0.3 volt drop concerns you? This actually puts us within the 1.0 watt limit per resistor at 0.92 watts.

Won't the 1.2 volt battery heat up slower than the 1.5 volt battery? The rechargeable batteries are so much more expensive than the normal 1.5 volt batteries.

Ivan Seeking

Nov15-04, 10:03 PM

The AA batteries looks good. You should take a look at the graphs on the second page here http://data.energizer.com/PDFs/e91.pdf and try to understand them. This is for a regular AA battery. With 8 batteries and about 0.12 amps, you should expect about ten good hours of knife-time per battery set.

Ivan Seeking

Nov15-04, 10:10 PM

[COLOR=Blue]

Should I get this?:

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=270-742

I would tend towards something like this
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=270-1238
or this
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=270-739

You want the fuses that go with the fuse holder that you purchase. This may be easier to do at the store. As long as the fuse is rated for at least 12 volts [which all will be] and a little more current than we need, in this case say 0.5 amps, that's all that you need.

Also, keep in mind that you only need one fuse but you want a few spares.

Ivan Seeking

Nov15-04, 10:18 PM

Won't the 1.2 volt battery heat up slower than the 1.5 volt battery?

Yes, as indicated by your power calculations. More power means quicker heating. This is not a great difference - 36% - but it would probably be noticable.

The rechargeable batteries are so much more expensive than the normal 1.5 volt batteries

Yes! :surprised :surprised :surprised

It really gets your attention doesn't it?

Ivan Seeking

Nov15-04, 10:23 PM

btw, your fuse could be as small as 0.2 amps if your circuit demands 0.12 amps. With a purely resistive circuit like yours you can cut this pretty close without causing any problems; ie without causing the fuses to blow too early. Other types of circuits require greater overhead.

Ivan Seeking

Nov15-04, 10:37 PM

oh yes...did I forget this??? Switches.

You need to decide if you want a momentary switch [must be held on] or one that latches on. How and where do you plan to mount it? It must be rated for at least twice the current used by your circuit, and the rated value must exceed the amp value chosen for your fuse. In other words, you want the fuse to blow before the switch is damaged. There is quite a selection here to choose from.
Search switches at Radio Shack (http://www.radioshack.com/category.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F002%5F015%5F000&Page=1&find=switches(keyword)&site=search)

Ivan Seeking

Nov20-04, 12:45 AM

Shay's deadline has passed.

Shay10825

Nov20-04, 09:39 AM

I'm sorry I forgot to tell all of you. I found out on Tuesday that the new dedline is December 8th. At first they wanted me to send in the report to them on November 18. Now they are just saying have the report and project at the science fair which is December 8th.

Shay10825

Nov21-04, 06:08 PM

I bought all of this today. Is this ok?
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F002%5F014%5F003&product%5Fid=271%2D152&site=search

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F002%5F015%5F006&product%5Fid=275%2D614&site=search

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F004%5F004%5F000&product%5Fid=270%2D1238&site=search

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F004%5F003%5F000&product%5Fid=270%2D1047&site=search

Shay10825

Nov21-04, 07:19 PM

What do I mount the switch on?

Cliff_J

Nov21-04, 10:00 PM

I think something like this would work much better as suggested by Ivan above:

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=275-609

The rest looks fine as long as the 100ohm resistors will get warm enough to do the job.

Cliff

Shay10825

Nov26-04, 07:38 PM

I have a couple questions.

1.) how do I hook up the switch?
2.) how do I hook up the fuses and fuse holder?
3.) What do you think I should do to the back of the blade? I think I night cover the resistor with really thick aluminum foil. Do you think this would work?

I'm going to use the 10 ohm 1 watt resistor because the 100ohm did not get hot enough.

I hope you all had a great Thanksgiving :smile: !

Ivan Seeking

Nov27-04, 12:29 AM

You should use your ohm-meter to check the function of the switch which simply makes and breaks the connection in one of the two wires from your batteries to the resistor. It is standard to switch the positive side [to make and break the connection in the positive wire].

The fuse should be in-line with the switch. It acts just like the switch - making and breaking the positive side of the circuit - but as a failsafe rather than a regular switching device. The fuse should be the first device after the batteries.

As for your choice in resistors, you had better check your calculations. You have a problem.

Moonbear

Nov27-04, 01:17 AM

Shay, do you have a digital camera? I'd love it if you'd post a picture of your knife when it's completed! I'm sure with so many people contributing ideas to its design, they'd also enjoy seeing the final product.

Math Is Hard

Nov27-04, 01:42 AM

I agree. Can you have some pictures taken while you're at the science fair demonstrating it? That would be so cool!

Shay10825

Nov27-04, 09:13 AM

As for your choice in resistors, you had better check your calculations. You have a problem.
I'm using the 10 ohm 1 watt resistor and 4 batteries not 8.
Do you think the aluminum foil would work?

Shay, do you have a digital camera? I'd love it if you'd post a picture of your knife when it's completed! I'm sure with so many people contributing ideas to its design, they'd also enjoy seeing the final product.
I agree. Can you have some pictures taken while you're at the science fair demonstrating it? That would be so cool!
Yes I'll post the pictures :smile: .

Shay10825

Nov27-04, 04:07 PM

I bought a momentary switch. The switch has two metal loops. Should I connect the positive wire comming out of the battery holder to one loop and one end of the fuse holder to the other?

Ivan Seeking

Nov27-04, 05:11 PM

Oh good I was going to ask about pictures. :biggrin:

As a good practice the fuse should come first in line, then the switch, but you have the idea. The switch just makes the connection to complete the circuit. The "positive current" flows from the positive terminal of the battery, through the fuse, through the switch, then through the resistor and back to the negative battery terminal.

The foil should work okay. Be careful not to create a short circuit by making contact with the resistor's bare leads. Also, make sure that you apply a good layer of heat conductive paste to the body of the resistor, and the points of contact with the blade and foil. Since you have chosen to risk over-driving the resistor you need to pull the heat away from the resistor's body as fast as possible. The foil will help to do this.

Note that 3 watts of heat is just that, 3 watts of heat. In principle it doesn't matter if we get 3 watts from 1, or from 4 resistors. Are you having trouble trying to fit more than one resistor to the blade? How are you securing the resistor?

Shay10825

Nov27-04, 08:24 PM

would i connect it like this?
http://img47.exs.cx/img47/7294/projcombo.png

Sorry that my picture really sucks :biggrin:

Shay10825

Nov27-04, 08:54 PM

Shay10825

Nov27-04, 09:15 PM

Would it look bad if the blade was shaped like this?
http://img109.exs.cx/img109/9249/blade.png

Ivan Seeking

Nov27-04, 09:48 PM

The circuit looks correct. The blade looks fine to me as well.

We know how you feel. Welcome to the world of invention and design. I'll bet you never thought that engineering and science could be painful. :biggrin:

Gokul43201

Nov27-04, 10:39 PM

:cry: This blade is driving me crazy!!!!!!! I cut the sheet metal and it's not turning out right. The edge of the blade I cut is jagged and it does not really look like a knife blade. I filed down the edges of it but it still has not turned out right. I don't think it really looks like a blade. The blade is not perfectly flat either. I've cut myself a million times :cry: (not purposely). Does anyone have any suggestions on what I can do.

What did you use to cut it ? Ideally, you could cut it with a shears (if it's less than1/16"). Any machine shop/metals supplier will do that for you.

What metal are you using, and how thick is it ?

Shay10825

Nov28-04, 07:32 PM

I cut it with shears. It is aluminum sheet metal and it's 1 mm thick.
I don't know what I'm going to do about the blade. I don't like how it looks with the foil on the back covering the resistors. If I put another piece of sheet metal what would I glue it together with. I was going to use liquid nails but it is so messy. When I put the glue on the metal the two pieces of metal did not want to stick together. It just became a big mess because the glue got all over the resistor and heat sink. So I don't want to use glue. What else could I use if I want to stick the two pieces of metal together? Could I weld it?

I also went to look for something to cover the battery holder. I wanted something like this:
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=270-1803
but I could not find anything that the battery holder I have would fit in(the one shown is way too tall). So I was just walking around Home Depot thinking about what I could do and I passed the plumbing isle. So I bought a pipe that the battery holder can fit in. I was also in the craft store today and I got some wood. Which one do you think would look better covering the battery holder? Should I use the wood and make a little box, or should I use the white plastic pipe?

Ivan Seeking

Nov28-04, 08:39 PM

First, you had better not set yourself up by using an under-rated resistor and then preventing easy access for needed replacements. You need to plan for a resistor failure.

What about using a piece of copper tubing instead of a blade? This might be considered a spreading but not slicing device. Maybe you could attach a cutter to the tube?

Otherwise, I'm thinking.

Ivan Seeking

Nov28-04, 08:42 PM

You could use one piece of metal and fold it over - to fit the resistor and wires but still approximate a blade. This would leave only one long side with edges. One edge could be folded around the opposite edge and sealed.

Ivan Seeking

Nov28-04, 08:56 PM

You could put the resistor[s] in a small copper tube [maybe 1/8" to 1/4" diameter] after attaching the tube to the top of the blade. You could braze or solder using lead free alloy such as silver solder.

Shay10825

Nov28-04, 09:27 PM

One edge could be folded around the opposite edge and sealed.

This is a really good idea :smile: . I was folding one piece of metal but I did not think of folding it around the opposite edge also. This might work. But what should I seal it with?

Shay10825

Nov28-04, 09:30 PM

Ivan Seeking

Nov28-04, 09:41 PM

Just make sure that you don't braze or solder with the resistor[s] in place. Those must be inserted afterwards or they will be damaged.

On another note and as a reminder, four resistors producing 3 watts of power at a relatively low temperature produce the same amount of heat energy as one resistor producing 3 watts of power at a relative high temperature. As long as you have a good thermal bond between the resistors and the metal [by using the heat sink compound], and if in an otherwise closed system like a closed metal tube, the heat produced in the metal tube is the same. It should take a little longer to get warm since we have more resistors to heat up [also, as long as the blade isn't too massive or too large], the difference in the heating shouldn't be very great.

In this case, power is the measure of energy lost to heat in the resistor per second. So, since all of the power is going to heat, the watts of power produced is what matters, not how we produce it.

Ivan Seeking

Nov28-04, 09:49 PM

You might even pound, press, or somehow crimp a large, thin wall copper tube into a blade.

Moonbear

Dec1-04, 10:49 PM

If you're not brazen enough to braze or solder, the glue/adhesives section in Home Depot ought to have some plumbing epoxy type materials. There are a bunch of these for different applications. Find one that will bond metals and is okay to get wet (though, don't get that blue marine adhesive they sell...I've tried that and it peels even when it shouldn't). The two-part epoxys usually come in a tube with two plungers that mixes them as they come out. Use in a well-ventilated area and be careful not to glue your fingers to the knife. They are usually pretty thick, so won't run all over the place out of control.

Where on the blade are you attaching the resistors? Near the top?

I think diagrams help to be sure everyone is talking about the same thing. This is a quick sketch of what I think you guys are talking about for the blade and cover for resistors, but I might be misunderstanding. See if this helps or is actually what you are all trying to accomplish.

Moonbear

Dec6-04, 08:38 PM

Just two more day's until Shay's science fair. Good Luck Shay!!! Knock 'em dead! And don't forget the pictures. :smile:

Ivan Seeking

Dec6-04, 09:56 PM

Just two more day's until Shay's science fair. Good Luck Shay!!! Knock 'em dead! And don't forget the pictures. :smile:

Hopefully, no news is good news. :cool:

Shay10825

Dec10-04, 08:04 PM

Hi everyone :smile: !!! The science fair was sooooooo much fun!! The judges really liked my project. I had 8 judges for some reason (we were only suppose to have 4). I had more judges than anyone there ( I don't know if this is a good thing or not). They said the title made them wonder what it was :rofl: .

Guess what???

I won 1st place :biggrin: !!!

You want to know why? :biggrin: :rofl: :rofl: I was the only person in the inventions category :rofl: :rofl: :biggrin: .

But it was a lot of fun. I really enjoyed it.

I'll post the pictures as soon as I get them developed.